This invention relates to a focus control device which is applicable to a pickup in an optical video disk player or digital audio disk player.
FIG. 7 is an explanatory diagram of a conventional pickup of this type according to a so-called "critical angle method". In FIG. 7, reference numeral 1 designates a critical angle prism; 2, a light receiving element which is divided into two light receiving element units; and 3, a differential amplifier. In the system, when a focusing state is obtained, rays of light incident to the critical angle prism 1 are parallel and reflected at an angle close to the critical angle. Accordingly, when a disk (not shown) is at the focusing position of an objective lens (not shown), the quantities of light applied to the two light receiving element units are equal to each other. However, in the case where the disk and the objective lens are moved towards each other, the light beam on the right side of the optical axis in FIG. 7 is applied to the critical angle prism 1 at an angle smaller than the critical angle; and in the case where the disk and the objective lens are moved away from each other, the light beam on the left side of the optical axis is applied to the critical angle prism 1 at an angle smaller than the critical angle. Therefore, in each of the cases, the light beam passes through the prism 1, and the output of the upper or lower light receiving element unit is smaller, as a result of which the differential amplifier 3 provides an output. The position of the objective lens is controlled according to the output thus provided; that is, the focus controlling operation is carried out.
Focus control devices according to an astigmatism method and a knife edge method are also known in the art.
However, the focus control device according to the critical angle method is disadvantageous in that its manufacturing cost is high and the pickup itself is heavy, because it uses the critical angle prism. The focus control device according to the astigmatism method or knife edge method is also disadvantageous in that it a bulky and intricate in adjustment because it must be operated under the condition that the light beam converges.